The Merton model views firm equity as a call option on the firm's assets, with one of its core assumptions being that firm asset value follows a Geometric Brownian Motion (GBM). However, in reality, firm asset values can experience sudden, discontinuous jumps, especially in the face of macroeconomic shocks or specific company events. Discuss which core assumptions of the Merton model are challenged under a jump-diffusion framework. How would you modify or extend the Merton model to better capture these jump behaviors, and what are the implications for valuation and credit risk management?

Merton模型将公司股权视为对公司资产的看涨期权，其核心假设之一是公司资产价值遵循几何布朗运动（GBM）。然而，现实中公司资产价值可能经历突发性、跳跃式变化，尤其是在面对宏观经济冲击或特定公司事件时。请讨论在这种“跳跃扩散”机制下，Merton模型的哪些核心假设会受到挑战？你将如何修正或扩展Merton模型以更好地捕捉这些跳跃行为，并评估这对估值和信用风险管理的影响？